The CIA World Factbook claims that the Caribbean island of Saint Martin is “the smallest landmass in the world shared by two independent states.” I don’t want to pile on the CIA, which has had a pretty rough decade, but agency's fact is only true if you pretend that the overseas Dutch and French territories on Saint Martin are independent states, and ignore a handful of other, much smaller islands situated on the boundaries between two neighboring mainland countries. Perhaps the smallest of these is the very unusual Baltic Sea islet called Märket.

Märket is a barren, eight-acre lump of granite between Sweden and the Åland Islands of Finland, only 500 feet wide. The Russians and Finns built a lighthouse on the island in 1885, but it has run automatically since 1979—no one’s lived on the island for more than 30 years, apart from a colony of seals. After the Finnish War of 1809, Sweden ceded the Åland Islands to the Russian Empire, and the new border divided Märket exactly in half. No border markings could withstand the ferocious Baltic wind and waves, so ten holes drilled into the rock are the only sign of an international border there.

Despite its small size, Märket has one of the world’s oddest borders: a big backward letter “S” almost looping back on itself twice as it bisects the island. The border was a straight line until 1985, when better maps and surveying revealed that the island’s only landmark, the Finnish lighthouse, had actually been built on the Swedish half! The border had to be redrawn without giving more land to either country or affecting the fishing rights on the coastline, which explains its tortured (but symmetrical) shape. A narrow Finnish passage, only 75 feet wide or so at one point, connects the lighthouse to the Finnish side of the island.

Many amateur radio operators are enthusiastic “collectors” of all the world destinations they’ve contacted via ham radio. In 1969, some Finnish devotees realized that tiny Märket actually qualified as a separate “country” under the rules, because the Åland Islands separate it from Finland proper. They began organizing regular trips to the barren island to set up an antenna and man a station there. “Market Reef,” as it’s called on official ham lists, is still one of the rarest and most eagerly sought-after contacts in the radio world.

Keith Clarke, the geography professor who writes the Ask Dr. Map column for the American Congress on Surveying and Mapping, has called Märket the world’s smallest island divided between two countries. In fact, there are smaller divided lake and river islands, such as a three-acre island in Boundary Lake that straddles North Dakota and Manitoba, but Märket may be the smallest sea island. Or is it? I’ve seen a Finnish map that divides Koiluoto, an even tinier island in the Gulf of Finland, in half between Finland and Russia. On Google Earth, though, the border misses the island by 50 feet or so, which seems more likely. We may never know who owns Koiluoto until someone tries to build a lighthouse there.

Explore the world's oddities every week on CondeNastTraveler.com with Ken Jennings. Check out his latest book, Maphead.